FPGEE for National Association of Boards of Pharmacy (NABP) Practice Exam

What formula is used to calculate the loading dose of a drug?

1. Loading dose (mg) = [Cp (mg/L) x Vd (L)] / F

2. Loading dose (mg) = [Cl (L/hr) x Vd (L)] / Cp

3. Loading dose (mg) = [Cp (mg/L) x F] / Vd

4. Loading dose (mg) = [Cl (L/hr) x F] / Cp

The correct answer is: Loading dose (mg) = [Cp (mg/L) x Vd (L)] / F

The loading dose of a drug is calculated using the formula that takes into account the desired plasma concentration (Cp), the volume of distribution (Vd), and bioavailability (F). The purpose of a loading dose is to achieve a therapeutic concentration of the drug in the bloodstream as quickly as possible. The correct formula defines the loading dose in terms of these three critical parameters. The desired plasma concentration is multiplied by the volume of distribution to determine the total amount of drug required to reach that concentration. This value is then divided by the bioavailability factor to account for how much of the drug actually reaches systemic circulation after administration. In essence, this formula allows for adjusting the dose based on the drug's pharmacokinetics and the patient's individual characteristics, ensuring that an adequate drug level is achieved rapidly. When considering the other formulas presented, they do not correctly reflect the relationship of the desired concentration and the volume of distribution necessary to determine the loading dose: one formula incorrectly positions values, and others involve the clearance concept which is not directly applicable for calculating loading dose. This specificity makes the chosen formula the most accurate for determining how much drug to administer initially to quickly achieve therapeutic levels in the patient.